1. Field of the Invention
The invention relates to a method for forming chemically bonded phosphate ceramics (CBPCs) of metal oxides and more specifically, this invention relates to a method for using trivalent oxides of metals to create a ceramic.
2. Background of the Invention
Haematite, having the chemical formula Fe2O3, is one of the most abundant minerals in nature. It exists as iron ore, in other minerals such as bauxite, and is also a component in clay minerals. It is the major component in laeritic soils (red soils found in the tropics). Similarly, manganese oxide, having a formula Mn2O3 is also a very common component in several lateritic soils and also exists as a mineral of manganese in the tropics.
When iron rusts, haematite is produced. Haematite also exists in metal machining wastes called swarfs, and in slags from steel industry. Iron mine tailings, containing haematite, are the most voluminous wastes that occupy land and pollute air with their dust. Thus haematite is abundant as a mineral as well as a waste and is a very inexpensive raw material. If a method is developed to bind haematite powder at ambient temperature by chemical reactions to form ceramics, the ensuring structural products can be very cost effective. This is particularly relevant in light of rising metal prices due to exhaustion of ore sites, and energy costs. This is because such a process will reduce energy consumption in the production of sintered structural products such as construction products and components.
Typically, tin chloride (SnCl2) or FeS are used to reduce haematite to iron oxides such as magnetite (Fe3O4) or to worstite (FeO). For example, U.S. Pat. No. 6,133,498 issued to Singh et al. on Oct. 17, 2000 disclosed a method to produce chemically bonded phosphate ceramics for stabilizing contaminants encapsulated therein utilizing reducing agents. The iron oxide is then reacted with phosphoric acid solution to form ceramics.
These reducing agents have their drawbacks. First, tin chloride is very corrosive. Its chloride anions released in the reacted slurry tend to interfere with the setting reaction, thereby weakening the product. Also, the amount of tin chloride that needs to be added is a relatively high 5 wt.% of the total amount of haematite powder used. With its high cost, this component raises the cost of the product significantly so that any advantages of lower costs of the haematite are lost.
FeS releases sulfur dioxide (SO2), which is an atmospheric pollutant. The gaseous SO2 also generates additional porosity in the material and therefore weakens it. Furthermore, like tin chloride, FeS can be costly, resulting in a more expensive product. The ""498 patent does not provide an alternative to the use of SnCl2 and/or FeS.
The U.S. Pat. Nos. 5,645,518 and 5,830,815 issued to Wagh et al. on Jul. 8, 1997 and Nov. 3, 1998, respectively, disclose processes for utilizing phosphate ceramics to encapsulate waste. U.S. Pat. No. 5,846,894 issued to Singh et al. on Dec. 8, 1998 discloses a method to produce phosphate bonded structural products from high volume benign wastes. None of these patents provides a method for utilizing the waste materials of iron and manganese.
A need exists in the art for a method to produce low cost structural products using inexpensive iron and manganese oxides. These oxides would be available either as waste materials or as natural minerals and the ceramics generated therewith could be used to encapsulate metal wastes that require reduction environments such as hazardous materials (e.g. chromium and arsenic), fission products such as technetium wastes and low-level radioactive materials. The method should result in the formation of a durable and chemically stable ceramic. Also, the method should utilize inexpensive and commonly available reactants to produce low cost ceramics.
An object of the present invention is to provide a method of producing chemically bonded phosphate ceramics (CBPCs) that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a room temperature, non-toxic method for incorporating trivalent metal oxides in a ceramic. A feature of the invention is that elemental metal, or minerals and waste oxides containing the elemental metal, can be used to reduce the oxides. An advantage is that the oxide materials are abundant and therefore inexpensive.
It is another object of the present invention to provide a method of reducing haematite that does not use expensive, corrosive or polluting materials. A feature of the invention is that elemental iron is used to reduce haematite to more soluble species. An advantage of this feature is that the species can be utilized as a ceramic binder to encapsulate hazardous and radioactive waste.
Briefly, the process provides a method for forming ceramics from metal oxide, the process comprising supplying a mixture of elemental metal and the oxide, reacting the mixture with acid, and adding a retardant to the mixture to slow the setting rate.
Also provided is a method for stabilizing hazardous and radioactive waste by the formation of CBPC""s and incorporating the waste materials in either the haematite powder compositions or in the phosphoric acid solutions.
Finally, a method is provided for retarding the setting rate of Ferroceramicrete so as to facilitate its use in industrial settings.